CVD reactors of the type mentioned above are well known in the prior art. The invention thereby takes as a starting point a CVD reactor such as is shown in FIG. 3. The shown CVD reactor comprises a cylindrical, vertical reactor chamber 10 that is delimited by a reactor wall 11 and a reactor base 12. The reactor wall 11 is dome-shaped and comprises a hollow cylindrical section and a dome 14 facing away from the reactor base. A plurality of heaters 1 to 4 are provided along the reactor wall 11, by means of which the reactor wall 11 can be heated. Suitable end parts 15 can be found in the upper and lower regions. The reactor is therefore basically disposed in a homogeneously heated space 16.
The shown CVD reactor furthermore comprises a central inlet line 17 for the continuous inflow of reaction gas. The central inlet line is formed by a central pipe that can be rotated via a drive 18. Downstream of the site where the inlet line 17 passes through the reactor base 12, a preheating chamber 19 is integrated in the inlet pipe, which chamber is formed by a container that has flow diversions and/or baffles (not shown) provided therein. Downstream of the preheating chamber 19 a plurality of gas outlets 20 are provided in the central inlet pipe.
A centrally arranged, tiered workpiece receiving element 21 is furthermore disposed in the reactor chamber 10. It comprises a plurality of tray-shaped receiving elements that are arranged one above the other. The tiers are respectively formed between two receiving elements. The gas outlets 20 of the central inlet pipe 17 are each arranged at the level of a tier and open out into the respective tier above the workpiece tray. At their radially outer end, the respective tiers are in fluid connection with the reactor chamber 10. An end plate 22 is furthermore provided above the uppermost tier.
The central inlet pipe 17 that can be rotated by means of the drive 18 is rotatably mounted in the tiered workpiece receiving element 21.
Furthermore, an outlet line 23 passes through the reactor base 12 and forms an outlet for used reaction gas out of the reactor chamber 10.
There are several problems with respect to this solution. On the one hand, the preheating chamber 19 has limits with respect to its surface that comes into contact with the inflowing reaction gas, i.e. if the preheating chamber per se is not supposed to be enlarged, the surface can only be enlarged with difficulty and combined with the integration of further flow-impeding baffles. Efforts are being made to increase the amount of inflowing gas (reaction gas) and to thereby shorten the coating process and/or to keep the thickness of the layer within narrow limits over the reactor volume. However, if the amount of gas is increased, the heating capacity of the preheating chamber must also be increased. Furthermore, part of the reactive gas mixture is prematurely consumed on the inner surfaces.
Furthermore, owing to the flow diversions in the preheating chamber the speed of the reaction gas is reduced. As a result, there is the added problem that in the preheating chamber a certain deposition process already occurs on the surfaces of the preheating chamber. This is a problem in particular in the case of reaction gases that contain Lewis acids and Lewis bases. Deposition problems are often encountered therewith. One example is the system TiCl4/CH3CN. The preheating chamber that is disposed in the rotatable central inlet pipe 17 and that rotates therewith is complex to dismantle and take apart since the premature coating welds the parts of the chamber together, and thus cleaning of the preheating chamber, i.e. the removal of the deposits in the preheating chamber, is problematic and time-consuming. In the case of very reactive gas mixtures, a thick layer formation can occur locally which, in extreme cases, leads to blockages.
Finally, it is also difficult with the preheating chamber to precisely regulate and control the temperature of the gases to be preheated. Another major disadvantage is that all reactive gases must be mixed prior to the rotating preheating chamber.
As is apparent from FIG. 3, the reaction gas is fed into the central inlet pipe 17 via a stationary gas inlet that is connected to the reactor base 12. The central inlet pipe 17 is rotated by means of the drive 18. This results in an interface between the stationary gas inlet and the central inlet pipe 17. This interface is formed in the region of the reactor base 12 and causes considerable problems in particular as regards sealing so that the escape of reaction gas at said interface and thus out of the reactor cannot be completely avoided in particular owing to the support of the seal at the specified high temperatures. This is a serious problem especially in the case of highly reactive, in particular corrosive starting substances.
When using starting products with low vapor pressures, this point must be heated to temperatures of greater than 200° C., which requires an expensive and complex technique for sealing the rotating duct.
Known CVD reactors do not provide any solutions to the aforementioned problems. DD 111 935 discloses, for example, the downward introduction of a reaction gas via a central inlet pipe that is provided with gas outlets and that enters the reactor chamber in the region of the reactor lid. It is furthermore proposed herein to cool the central inlet line. The latter is in stark contrast to the specification to preheat the reaction gas before introduction into the reactor chamber. Furthermore, DD 111 935 explicitly teaches a flow outwards from the central inlet pipe towards the reactor wall so as to prevent an increased deposition rate on the reactor wall.
DE 197 43 922 proposes the continuous switching of the flow of reaction gas into the reactor chamber. Therein, the reaction gas flows outside a central pipe, over the workpieces to be coated, into the central pipe via an opening in the central pipe that is facing away from the reactor base and then out of the reactor, or—after switching—the reaction gas flows in the reverse direction upwards through the central pipe, out of the opening facing away from the reactor base and from there downwards over the workpieces to be coated and out of the reactor chamber. A relatively complex two-way valve arrangement that has to withstand high temperatures is provided herefor, and furthermore, the integration of gas preheating and mixing before coating of the workpieces is difficult, in particular when reactants with low vapor pressures are present, since the preheating would have to be carried out at different outlets depending on the different valve positions.
EP 0 164 928 A2 relates to a vertical hot-wall CVD reactor having two parallel inlet and outlet lines disposed in the edge regions, said lines passing through the base of the reactor into the reactor chamber and extending from there up to the uppermost tier of a tiered workpiece receiving element. The workpiece receiving element is rotatably mounted. The substrates to be coated are thereby supposed to be placed in the centre of the rotational axis of the workpiece receiving element and thus precisely centrally between the inlet and outlet lines.
US 2006/0159847 A1 comprises two inlet pipes that presumably extend through the base and from there up to the uppermost tier of a workpiece receiving element. The workpiece receiving element itself is not rotatable. The inlet lines can rather be rotated about 360°, and the workpiece receiving element is herein also supposed to be arranged centrally between the inlet and the outlet.
US 2007/0246355 A1 also discloses an inlet line that extends from the base up to the uppermost tier as well as an outlet line in the region of the base. A rotatable workpiece receiving element is furthermore provided.
With respect to EP 0 270 991 B1, a stationary inlet line is arranged centrally, i.e. in the middle relative to the reactor chamber and is surrounded by a rotatable outlet line.
Furthermore, two workpiece receiving elements are disposed to the left and right of the inlet and outlet line and are accommodated in an inner reactor chamber that is in turn surrounded by an outer reactor chamber. The central outlet line is rotatable. If this outlet line is rotated, the base plate rotates therewith and takes with it the workpiece receiving elements such that these workpiece receiving elements rotate about the outlet line. Owing to this rotation and to the fact that the toothed wheels engage with the fixed toothed wheel, the workpiece receiving elements themselves rotate about their central axes.
DE 78 28 466 U1 relates to the coating of workpieces, in particular tools. A central inlet line is rotated here, but not, however, the stationary workpiece receiving element.